1. Field of the Invention
This invention relates to a process for producing highly water absorptive polymers which are obtained in a spherical form having a large particle size, have a very narrow particle size distribution, are free-flowing due to very little content of residual organic solvent and are of extremely high fluidity.
The highly water absorptive polymers obtained by the process of this invention are insoluble in water and are swollen by absorbing much water. The polymers are in a spherical form having a large particle size with a very narrow particle size distribution. Further, the polymers are free-flowing due to a very small content of organic solvent residues and are thus of extremely high fluidity and, therefore, they can be easily handled with great safety. Accordingly, the polymers obtained by the process of the present invention are advantageously used for various absorbent materials and various materials which are used in the swollen state with water being absorbed.
2. Background Art
Reverse-phase suspension polymerization techniques for producing highly water absorptive polymers by polymerizing a water-soluble ethylenically unsaturated monomer in the dispersion phase of an aqueous monomer solution in the continuous phase of an organic solvent substantially incompatible with the aqueous solution phase and in the presence of a dispersant have been generally well known in the art, and some polymerization techniques characterized by the types of dispersant and organic solvent used, polymerization conditions and other factors have been reported. For instance, there have been known techniques in which as the dispersants use are made of:
nonionic surfactants having an HLB of 3-6 (see Japanese Patent Publication No. 54-30710 and Japanese Patent Kokai Publication No. 61-157513); PA1 nonionic surfactants having an HLB of 6-9 (see Japanese Patent Kokai Publication No. 57-167302); PA1 nonionic surfactants having an HLB of 8-12 (see Japanese Patent Publication No. 60-25045); PA1 polyglycerin fatty acid esters having an HLB of 2-16 (see Japanese Patent Kokai Publication No. 62-172006); PA1 sucrose fatty acid esters (see Japanese Patent Kokai Publications Nos. 61-87702 and -43606); PA1 cellulose esters or ethers (see Japanese Patent Kokai Publication No. 57-158210); PA1 methacrylic acid-methyl methacrylate-2-ethylhexyl acrylate copolymers, products obtained by the reaction of maleic anhydride-modified polybutadiene with 2-hydroxyethyl methacrylate, polybutadiene-butyl methacrylate-methacrylic acid graft copolymers, ethylenepropylene-diene monomer copolymers grafted by .alpha.,.beta.-unsaturated carboxylic acids and alkyl esters or hydroxyalkyl esters of .alpha.,.beta.-unsaturated carboxylic acids, and so on (see Japanese Patent Kokai Publication No. 57-94011); and PA1 copolymers of .alpha.-olefins with .alpha.,.beta.-unsaturated polyvalent carboxylic anhydrides or their derivatives (Japanese Patent Kokai Publication No. 62-95308).
The polymers obtained by these methods take the form of a relatively large sphere, as expressed in terms of a primary particle size of a few .mu.m to a few hundreds .mu.m, but they have a relatively wide particle size distribution, because the so-called "fine powdery polymer" having a particle size of a few tens .mu.m or below are unavoidably produced. Thus, such dust-preventing measures as used in handling dust are needed. It is also generally said that as the content of the fine powdery polymer increases, the so-called "gel blocking" phenomenon is more likely to occur in absorbing the fluid to be treated, thus rendering such absorption very difficult. In addition, when such water-absorbing polymers are mixed with and retained by pulverized pulp, unwoven fabrics or the like, for example, with a view to using them as paper diapers or sanitary napkins, there is a problem that the fine powdery polymer tends to peel off.
In addition, it is unavoidable that the polymers obtained by the above-mentioned reverse-phase suspension polymerization methods contain the organic solvent used in the polymerization generally in an amount of a few tens to a few hundreds weight ppm or, in the worst case, as high as a few thousands weight ppm. Thus, such water-absorbing polymers present a serious safety problem, esp., when used as the materials for sanitary goods such as paper diapers or sanitary napkins.
Furthermore, when such a large amount of organic solvents remains in a polymer product, the powder particles are generally more likely to stick to each other. This presents a grave problem that their fluidity deteriorates when they are transported in the powdery form; it is impossible to transport them in a quantitative manner, for example in the production of paper diapers or sanitary napkins.